Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks, divided into sectors. Information is written to and read from a disk by a head (or transducer), which is mounted on an actuator arm capable of moving the head radially over the disk. Accordingly, the movement of the actuator arm allows the head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the head to access different sectors on the disk. The head may include separate or integrated read and write elements.
A disk drive 10 is illustrated in FIG. 1. The disk drive comprises a disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16.
The disk drive 10 also includes an actuator arm assembly 18 having a head 20 (or transducer) mounted to a flexure arm 22, which is attached to an actuator arm 24 that can rotate about a bearing assembly 26 that is attached to the base plate 16. The actuator arm 24 cooperates with a voice coil motor 28 in order to move the head 20 relative to the disk 12. The spin motor 14, voice coil motor 28 and head 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device. Instead of a one disk configuration (shown in FIG. 1), the disk drive 10 may include a plurality of disks 12 and, therefore, a plurality of corresponding actuator arm assemblies 18.
FIG. 2 is a diagrammatic representation of a simplified top view of a disk 12 having a surface 42 which has been formatted to be used in conjunction with a sectored servo system (also known as an embedded servo system). As illustrated in FIG. 2, the disk 12 includes a plurality of concentric tracks 44a–44h for storing data on the disk's surface 42. Although FIG. 2 only shows a relatively small number of tracks (i.e., 8) for ease of illustration, it should be appreciated that typically tens of thousands of tracks are included on the surface 42 of a disk 12.
Each track 44a–44h is divided into a plurality of data sectors 46 and a plurality of servo sectors 48. The servo sectors 48 in each track are radially aligned with servo sectors 48 in the other tracks, thereby forming servo wedges 50 which extend radially across the disk 12 (e.g., from the disk's inner diameter 52 to its outer diameter 54).
One of the operations that a disk drive performs is known as a seek operation. During a seek operation, the head 20 is moved from a present track of the disk to a target track of the disk, so that a data transfer can be performed with the target track. In order for a seek operation to be performed, a current is delivered to the VCM 28 of the disk drive, which causes the actuator arm 24 to rotate, thereby moving the head 20 radially relative to the disk surface 42.
It is desirable to perform seek operations as quickly as possible. Accordingly, in conventional long seek operations, for example, a maximum current will be applied to the VCM 28 in a first direction for a period of time to accelerate the head 20 towards a maximum velocity as it moves towards the desired track. Once the head 20 reaches its maximum velocity, no current is applied to the VCM 28 and the head 20 coasts at its maximum velocity for a period of time. Just prior to reaching the target track, in order to decelerate the head 20, a maximum current is applied to the VCM 28 in a direction opposite to the first direction, such that the head 20 is positioned near the target track. Once near the target track, the drive 10 may enter a linear mode to position the head 20 more closely to the target track. A diagrammatic representation of such a bang-coast-bang commanded current profile is illustrated as curve 302 in FIG. 3. (The actual current profile is illustrated as curve 304 and is different from the commanded current profile due to the back electromotive force (BEMF) associated with the actuator arm and VCM. This concept is well-understood to those skilled in the art.)
Application of currents in such a fashion causes abrupt changes in the acceleration and deceleration of the head. This tends to excite vibration modes in the drive, which can cause acoustic noise due to seek operations (also known as seek acoustics). If seek acoustics are not kept within acceptable levels, a disk drive may fail to meet qualification standards, which reduces drive yields and increases the overall manufacturing costs of disk drives.
The locations where large changes occur in the current supplied to the VCM are known as transitions (or seek transitions). As shown in FIG. 3, which illustrates a long seek, transitions occur when changing from an acceleration phase to a coast phase (Region A) and from a coast phase to a deceleration phase (Region B). As shown in FIG. 4, which illustrates a shorter seek than that shown in FIG. 3, the head goes from an acceleration phase to deceleration phase without a coast phase. Accordingly, transitions occur when changing from an acceleration phase to a deceleration phase (Region C). Unless measures are taken to smoothen the transitions, it is likely that vibration modes will be excited and acoustic noise will be generated.
While others have recognized that transitions in the seek current can cause acoustic noise, the solutions that have been offered have not been completely satisfactory. For example, others have used open-loop techniques by slew rate limiting or by fixed transition shapes that vary by seek length. A disadvantage of such open-loop techniques is that the control input cannot be adjusted to ensure convergence of the commanded and actual position once a transition begins. Furthermore, the open-loop techniques are selected by trial-and-error and are not automated.
Accordingly, it would be advantageous to provide a method and apparatus for reducing seek acoustics in a disk drive system by providing smooth, quiet and robust transitions in seek current. It would also be desirable that such a method and apparatus would include a feedback mechanism (i.e., closed-loop system), so as to ensure convergence of the commanded and actual position once the transition has begun. It would also be beneficial if the method and apparatus were automated.